The invention relates to the field of surface treatment and in particular to hydrophobic thermoplastic films for surface coatings and more particularly to films obtained by low-temperature drying of a latex containing neither volatile organic compounds nor coalescents.
In the context of the invention, the term xe2x80x9clatexxe2x80x9d should be understood to mean an aqueous dispersion of polymer particles which can be obtained by the emulsion polymerization of one or more monomers.
Many latices are intended for the building, paper, textile and leather industries and are used in the surface treatments of various materials, particularly as paints, varnishes, adhesives and mastics. More generally, such latices are required to have a minimum film-forming temperature, called hereafter MFT, as low as possible. The MFT is the lowest temperature at which the particles form a continuous film after the water has been evaporated.
When the polymers constituting the particles dispersed in a latex have a relatively low glass transition temperature, denoted by Tg, it is easy for the latex to form a film at room temperature but the films obtained are tacky and their mechanical integrity is poor. On the other hand, if the polymers have a high Tg the films will be of better quality but film formation at room temperature would require the addition of a coalescents. The latter is generally a volatile organic compound which evaporates during application and film formation, as described in U.S. Pat. No. 4,455,402 and U.S. Pat. No. 5,021,469.
For obvious environmentalxe2x80x94friendliness and cost-reduction reasons, it is desirable to develop containing neither volatile organic compounds nor coalescents, having a relatively low MFT and able to give films at low temperature which have no surface tack and are of good mechanical quality.
To achieve this objective, EP 466,409 proposes a solution based on a mixture of two latices, one having a low Tg, generally called a xe2x80x9csoftxe2x80x9d latex which providing film formation and the other having a high Tg generally called a xe2x80x9chardxe2x80x9d latex providing mechanical integrity. EP 609,756 describes a latex, prepared in two steps, having a xe2x80x9csoftxe2x80x9d core with a Tg ranging from xe2x88x925xc2x0 C. to xe2x88x9255xc2x0 C. and a xe2x80x9chardxe2x80x9d shell with a Tg of less than 50xc2x0 C.
Likewise, FR 2,090,483 describes a latex, obtained by two-step polymerization, consisting of 35 to 50% of a polymer, obtained in the first step, having a Tg of less than 10xc2x0 C. and of 50 to 65% of a polymer, obtained in the second step, having a Tg of greater than 60xc2x0 C., the first-step polymer being crosslinked and fastened to the second.